The present invention relates to an amplifier a gain of which can be changed.
FIGS. 6 and 7 show a prior-art gain control amplifier, respectively.
The gain control amplifier shown in FIG. 6 is configured to amplify a signal inputted from an input terminal 1 by an emitter-grounded transistor 3 and output the signal at an output terminal 2. The amplifier has a high-gain mode in which a power supply voltage is applied to a collector of the transistor 3 from a power supply 10 through a load 9 and a collector-emitter of a base-grounded transistor 4 and a low-gain mode in which the power supply voltage is applied to the collector of the transistor 3 from the power supply 10 through the load 9, a resistance 7, and a collector-emitter of a base grounded transistor 6. Reference number 11 denotes a bias power supply generating a bias voltage that determines an operation point of the transistor 3.
The high-gain mode or low-gain mode is selected by a selector switch 12.
To select the high-gain mode, the selector switch 12 is turned to contact 12a. This causes the bias voltage to be applied only to a base of the transistor 4 out of the bases of the transistors 4 and 6 from a bias power supply 13 and the power supply voltage to be applied to the transistor 3 through the transistor 4. The signal amplified by transistor 3 is outputted from the output terminal 2 through a branch 5.
To select the low-gain mode, the selector switch 12 is turned to a contact 12b. This causes the bias voltage to be applied from the bias power supply 13 only to the base of the transistor 6 and the power supply voltage to be applied to the transistor 3 through the transistor 6. The signal amplified by the transistor 3 is outputted from the output terminal 2 through a branch 8. The gain in the low-gain mode is controlled through the resistance 7.
The gain control amplifier shown in FIG. 7 is different from the one shown in FIG. 6 in that gain control is accomplished by controlling the current ratio between a branch 25 and a branch 27 and current control is accomplished by controlling a base voltage of a base-grounded transistor 6.
In particular, a signal RFin inputted through an input terminal 1 is amplified by an emitter-grounded transistor 3 and outputted at an output terminal 2. Reference number 11 denotes a bias power supply generating a bias voltage that determines an operation point of the transistor 3.
The amplifier has a high-gain mode in which a power supply voltage is applied to a collector of the transistor 3 from a power supply 10 through a load 9 and the collector-emitter of a base-ground transistor 4, and a low-gain mode in which the power supply voltage is applied to the collector of the transistor 3 from the power supply 10 through the collector-emitter of the base-grounded transistor 6.
Base voltage is applied to the bases of the transistors 4 and 6 from power supplies 29 and 30, respectively. Current control is accomplished by changing an output voltage of the power supply 30 to control the base voltage of the base-grounded transistor 6 and the current ratio between the branches 24 and 27 is controlled.
As an example circuit in which a gain control amplifier is used, there can be cited a head amplifier provided at a front end of a radio frequency receiver. As shown in FIG. 8, band-pass filters 709 and 710 are connected in input and output stages, respectively, of a head amplifier A in order to improve selectivity. Reference number 711 denotes a mixer circuit.
Because the band-pass filters 709 and 710 are designed to satisfy an required pass frequency property at predetermined input and output impedances, a gain of the head amplifier A is switched between high-gain and low-gain modes in accordance with electric field strength in an area in which the radio frequency receiver is used.
Therefore, it is required that the predetermined input and output impedances be maintained independently of switching between the high-gain mode and the low-gain mode.
However, in the gain control method in FIG. 6, the output impedance varies depending on the high-gain and low gain modes and therefore it is difficult to achieve output impedance matching in both of the high-gain and low-gain modes.
According to the gain control method shown in FIG. 7, in which the control of the current ratio between the branches 25 and 27 is accomplished by controlling the base voltage of the base-grounded transistor 6, it is difficult to control the gain because the gain changes by 1 dB when the base voltage of the transistor 6 changes by 0.02 volts, for example.
The present invention has been achieved to solve the problems of the prior art and it is an object of the present invention to provide a gain control amplifier in which the amount of variation in output impedance between the high-gain mode and low-gain mode is small and gain control can be performed easily and accurately.
To solve these problems, the gain control amplifier of the present invention has a configuration in which a load is provided in an output circuit of a first transistor, an output circuit of a second transistor is provided between the output circuit of the first transistor and the load, an input signal supplied to the input of the first transistor is amplified, and the amplified signal is taken out from a connection point between the load and the second transistor. In the high-gain mode, only a control bias for the second transistor out of the second transistor and a third transistor is turned on, and in the low-gain mode, control biases for both of the second and third transistors are turned on.
According to the first aspect of the present invention, there is provided a gain control amplifier in which a load is provided in an output circuit of a first transistor, an output circuit of a second transistor is provided between the output circuit of the first transistor and the load, the input signal supplied to the input of the first transistor 1 is amplified, and the amplified signal is taken out from a connecting point between the load and the second transistor, wherein an output circuit of a third transistor is connected in parallel with a series circuit of the output circuit of the second transistor and the load and switch means for turning on and off a control bias for the third transistor is provided, the switching means is turned off to turn on a control bias only for the second transistor out of the second and third transistors in a high-gain mode, and the switch means is turned on to turn on the control bias for both of the second and third transistors in a low-gain mode.
According to this configuration, the amount of variation in the output impedance between the high-gain mode and low-gain mode is small and an output current is determined by the size ratio between the base-grounded transistors, therefore the gain can be controlled easily and accurately without depending on the control bias.
According to the second aspect of the present invention there is provided gain control amplifier in which a load is provided in an output circuit of a first transistor, an output circuit of a second transistor is provided between the output circuit of the first transistor and the load, the input signal supplied to the input of the first transistor is amplified, and the amplified signal is taken out at a connecting point between the load and the second transistor, wherein an output circuit of a third transistor is connected in parallel with a series circuit of the output circuit of the second transistor and the load, control terminals of the second and third transistors are connected with each other through switch means, the switching means is turned off to turn on a control bias only for the second transistor out of the second and third transistors in a high-gain mode, and the switch means is turned on to turn on the control bias for both of the second and third transistors in a low-gain mode.
According to the third aspect of the present invention, in the gain control amplifier according to the second aspect, the switch means is formed with a MOS transistor and a resistance is connected in parallel with a control terminal of the third transistor.
According to the fourth aspect of the present invention, in the gain control amplifier according to any one of the first through third aspects, a resistance is connected in series with the output circuit of the third transistor.
According to the fifth aspect of the present invention, in the gain control amplifier according to any one of the first through fourth aspects, the second transistor and the third transistor are differently sized.